Quick Answer: ⚡ Quick Answer Most Ontario homeowners need 3,500–6,000 watts to cover essential circuits (furnace blower, fridge, sump pump, lights, phone charging). Whole-home coverage including electric stove or electric heat requires 12,000–20,000 watts. Or use our interactive calculator below to find your exact number.
⚡ Interactive Backup Power Calculator
Check the appliances you need to run during an outage. We'll calculate your total running watts, peak starting surge, and recommend the right generator size.
⚠️ Estimates only. Check appliance nameplates for exact wattage. Always consult a licensed electrician before installing backup power equipment.
Why Sizing Matters
An undersized generator will trip breakers, damage motors, or simply fail to start your furnace when you need it most. An oversized generator wastes fuel and costs more to run. Getting this right is the single most important step in backup power planning.
There are two wattage numbers you need to understand:
- Running Watts (Rated Watts): Power the appliance uses while running continuously
- Starting Watts (Surge Watts): Power spike required to start motor-driven appliances — usually 2–3× the running watts
Your generator’s rated watts must exceed your total running load. Its surge capacity must handle the single largest startup draw.
Step 1: Identify Your Essential Circuits
Start with the “must have” loads — the appliances that prevent property damage, health risk, or severe discomfort during a winter outage. 💡 Ontario Priority Order: Furnace blower → Sump pump → Refrigerator/freezer → Well pump (if applicable) → A few lights → Phone/device charging. Everything else is a bonus.
Step 2: Appliance Wattage Reference Table
Use this table to estimate your load. Actual wattage varies by model — check the nameplate on your appliances for exact figures.
| Appliance | Running Watts | Starting Watts | Priority |
|---|---|---|---|
| Gas Furnace (blower motor) | 600–800W | 1,400–2,400W | 🔴 Critical |
| Electric Furnace / Heat Pump | 5,000–15,000W | 7,500–20,000W | 🔴 Critical |
| Sump Pump (1/3 HP) | 800W | 1,300W | 🔴 Critical |
| Sump Pump (1/2 HP) | 1,050W | 2,150W | 🔴 Critical |
| Well Pump (1/2 HP) | 1,000W | 2,100W | 🔴 Critical (rural) |
| Well Pump (1 HP) | 1,500W | 4,000W | 🔴 Critical (rural) |
| Refrigerator (full size) | 150–400W | 800–1,200W | 🟠 High |
| Chest Freezer | 100–200W | 500–800W | 🟠 High |
| LED Lighting (per room) | 10–60W | Same | 🟠 High |
| Phone / Tablet Charging | 10–25W | Same | 🟠 High |
| Laptop | 45–90W | Same | 🟠 High |
| CPAP / BiPAP | 30–60W | Same | 🔴 Critical (if needed) |
| Security System | 5–15W | Same | 🟡 Medium |
| Internet Router / Modem | 10–30W | Same | 🟡 Medium |
| TV (55" LED) | 60–130W | Same | 🟡 Medium |
| Microwave | 600–1,200W | Same | 🟡 Medium |
| Electric Kettle | 1,500–1,800W | Same | 🟡 Medium |
| Coffee Maker | 600–1,500W | Same | 🟡 Medium |
| Garage Door Opener | 350–750W | 800–1,400W | 🟡 Medium |
| Electric Stove / Range | 3,000–8,000W | Same | 🟢 Nice to have |
| Electric Water Heater (40 gal) | 4,500–5,500W | Same | 🟢 Nice to have |
| Central A/C (3 ton) | 3,500W | 6,000–8,000W | 🟢 Nice to have |
| Window A/C (10,000 BTU) | 1,200W | 1,800–2,400W | 🟢 Nice to have |
| Washing Machine | 500–1,000W | 1,200–2,300W | 🟢 Nice to have |
| Electric Dryer | 5,000–6,000W | Same | 🟢 Nice to have |
Step 3: Calculate Your Load
Use this simple method:
- List every appliance you want to run simultaneously
- Add up all their running watts — this is your minimum generator rated capacity
- Find the appliance with the highest starting watts
- Your generator’s surge capacity must equal or exceed that starting watt figure
- Add a 20% buffer for safety and headroom
Example: Typical 3-Bedroom Ontario Home (Essential Circuits Only)
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Gas furnace blower | 700W | 2,000W |
| Sump pump (1/3 HP) | 800W | 1,300W |
| Refrigerator | 200W | 1,000W |
| LED lights (3 rooms) | 120W | 120W |
| Phone charging (×4) | 80W | 80W |
| Internet router | 20W | 20W |
| TOTAL | 1,920W running | 2,000W peak start |
| Result: A 3,000–4,000W generator handles this load comfortably. With 20% buffer: minimum 2,300W rated capacity. A Honda EU3000iS or Generac GP3500iO would be ideal. |
Example: Rural Ontario Home with Well Pump
| Appliance | Running Watts | Starting Watts |
|---|---|---|
| Gas furnace blower | 700W | 2,000W |
| Well pump (1/2 HP) | 1,000W | 2,100W |
| Sump pump (1/2 HP) | 1,050W | 2,150W |
| Refrigerator | 200W | 1,000W |
| Chest freezer | 150W | 600W |
| LED lights + devices | 200W | 200W |
| TOTAL | 3,300W running | 2,150W peak start |
| Result: A 5,000–6,500W generator covers this load. The Champion 5500W or Generac GP5500 would work well. Note: don’t run the well pump and sump pump simultaneously if possible — stagger them to reduce peak load. | ||
| 💡 Rural Ontario Tip: If your property has a well, consider a dedicated well pump generator circuit. Many rural homeowners in Renfrew County and Eastern Ontario run a separate smaller generator just for the well to avoid oversizing their primary unit. See our full guide: How to Power Your Well Pump During an Outage. |
Step 4: Choose the Right Generator Size
| Generator Size | Best For | Fuel Consumption (est.) | Price Range (CAD) |
|---|---|---|---|
| 1,000–2,000W | Camping, RV, phone charging, small appliances only | 0.5–1.0L/hr | $400–$900 |
| 3,000–4,000W | Essential circuits — furnace, fridge, lights, sump pump | 1.0–1.5L/hr | $700–$1,800 |
| 5,000–7,500W | Most Ontario homes including well pump | 1.5–2.5L/hr | $900–$2,500 |
| 8,000–12,000W | Large homes, electric heating circuits, workshop | 2.5–4.0L/hr | $1,500–$4,000 |
| 14,000–20,000W | Whole-home standby, electric heat, EV charging | 4.0–6.0L/hr | $5,000–$15,000 installed |
Portable Generator vs Standby Generator
| Feature | Portable Generator | Standby Generator |
|---|---|---|
| Cost (CAD) | $500–$4,000 | $8,000–$20,000+ installed |
| Starts automatically | ✗ Manual | ✓ Automatic |
| Requires fuel storage | ✓ Gasoline/propane | ✓ Propane/natural gas |
| Transfer switch required | Optional (recommended) | ✓ Included |
| Outdoor setup required | ✓ Every use | ✗ Permanent install |
| Power capacity | 1,000–12,000W typical | 7,500–22,000W typical |
| ESA permit required | Only if hardwired | ✓ Required |
| Best for | Essential circuits, budget buyers | Whole-home, convenience |
Battery Backup vs Generator: Which Is Better?
For many Ontario homeowners, especially those in urban/suburban areas or with noise/fuel storage concerns, a battery backup system (like the EcoFlow DELTA Pro or Bluetti AC300) can replace a portable generator for short outages (under 24 hours).
See our full comparison: Generator vs Battery Backup vs Solar: 2026 Comparison →
Our Top Generator Picks by Size
Honda EU2200i (2,200W) — Best Inverter Generator Under 3,000W
Ultra-quiet, fuel-efficient, and Honda reliable. Ideal for essential circuits in a smaller home or cottage. Note: won’t start a large furnace blower — check your motor’s starting watts first.
Generac GP5500 (5,500W) — Best Mid-Size for Most Ontario Homes
The workhorse choice. Covers furnace, well pump, sump pump, fridge, and lights simultaneously. Widely available across Ontario. Good parts availability from Canadian dealers.
Champion 7500W Dual Fuel — Best Flexible Fuel Option
Runs on gasoline or propane — a major advantage in Ontario where gasoline can be hard to find during extended outages. 7,500W running / 9,375W surge. Excellent for rural properties.
Frequently Asked Questions
What size generator do I need for a 3-bedroom Ontario home?
A 3-bedroom home with gas heat needs 3,500–5,500W for essential circuits. If you have electric heat, you’re looking at 12,000–20,000W. Most Ontario homeowners with gas heat and a sump pump are well-served by a 5,000–6,500W generator.
Do I need a transfer switch for a portable generator?
Not legally required for extension-cord use, but strongly recommended. A manual transfer switch (around $500–$800 installed) lets you safely connect to your panel circuits and eliminates back-feed risks to Hydro One workers. A licensed electrician must install it and pull an ESA permit.
How much fuel should I store for an extended Ontario outage?
A 5,500W generator uses approximately 2 litres of gasoline per hour at 50% load. For a 72-hour outage running 8 hours/day, that’s roughly 48 litres. Ontario allows up to 120L of gasoline storage in approved containers. Rotate fuel every 6–12 months and use fuel stabilizer.
Can a battery backup run my furnace?
Yes — a quality battery station like the EcoFlow DELTA Pro (3,600Wh) can run a gas furnace blower (700W) for approximately 3–4 hours. For overnight coverage, two units in parallel or the EcoFlow Power Station with expansion battery is needed. See our full comparison for details.